Recent electronic still cameras include a lens barrel that can be housed in a camera body when not in use in order to satisfy the requirement for compactness when the camera is not in use. The lens barrel and lens groups forming the lens system of the camera are supported in the camera to be movable along the optical axis. The lens barrel and the lens groups are extended to various positions toward an object being imaged when the camera is being used for imaging, and the lens barrel and lens groups are retracted to or toward positions inside the camera body when the camera is not in use.
For example, Japanese Laid-Open Patent Application No. 2001-242368 discloses a lens apparatus with a lens barrel and lens groups that are movable along the optical axis. First and second lens groups, in order from the object side, are movably supported in three-way suspensions by pins. Third and fourth lens groups, in order from the object side, are movably supported by guide bars. The lens apparatus may be retracted to a position where its retracted length is only 60 percent of its maximum extended length.
The lens apparatuses of recent electronic still cameras are also required to provide a bright image along with high zoom ratios. This, in turn, requires an increased number of lenses and lens groups. Additionally, an increasing number of lens apparatuses have been designed to house drive mechanisms such as motors in the lens barrel for further compactness. Especially recently, much higher zoom ratios have been demanded. However, the prior art structures hamper achieving a high zoom ratio with compactness.
For example, the lens apparatus described in Japanese Laid-Open Patent Application No. 2001-242368, discussed above, includes three lens frames that are movable by cam pins on their outer periphery that are engaged with forwarding grooves on a fixed barrel and with cam grooves on a cam barrel provided on the outer periphery of the fixed barrel. More specifically, three forwarding grooves for each of the fixed lens group and the zoom lens group are provided on the fixed barrel at regular intervals circumferentially around the periphery of the fixed barrel, and three cam grooves for each of the fixed lens group and the zoom lens group are provided on the cam barrel at regular intervals circumferentially around the cam barrel. Additionally, a forwarding groove for a relay lens group is provided on the fixed barrel, and a cam groove for the relay lens group is provided on the cam barrel.
With the above described structure, in order to obtain a zoom ratio of eight, the cam grooves are highly inclined so as to increase the movements of the lens groups. Thus, the cam grooves must be made more nearly parallel to the optical axis or the cam barrel must have a larger circumference in order to increase the movements of the lens groups. However, if the cam grooves are made more nearly parallel to the optical axis, the cam grooves receive an increased force in the normal direction and, therefore, the frictional drag between the cam groove and the cam pins increases, which can lead to malfunctioning of the lens apparatus. On the other hand, making the circumference of the cam barrel larger is counter to the requirement for compactness.
The present invention relates to a compact lens apparatus that provides a high zoom ratio that can be used in an electronic still camera.